This invention relates to switches for switching very large electric currents and has particular application to switches used in switching the very large currents employed in the electromagnetic propulsion of projectiles.
Electromagnetic projectile launchers have been constructed which include a pair of generally parallel conductive projectile launching rails, a sliding conductive armature for conducting current between the rails and propelling a projectile along the rails, a source of high current and a switching system for switching current from the current source to the rails. Firing a burst of electromagnetically accelerated projectiles will generally require prestorage of energy which is depleted by the successive energy requirements of each projectile acceleration. Two distinctly different systems have been proposed. Each involves the prestorage of energy in the form of the kinetic energy of a rapidly revolving rotor of an electrical generator.
One system involves a homopolar generator-inductor system in which a homopolar generator charges an inductor to a firing current level and suitable switching then fires a projectile after which the charging circuit is reestablished so that the inductor can be rapidly recharged back to the same firing current level for a successive shot. The firing switch or switches for this type of operation commutate the projectile accelerating current into the breech end of the projectile rails and therefore the firing switch or switches conduct current continually except during the very brief successive projectile acceleration intervals. Thus these switches must, without overheating and deterioration, accommodate enormous magnitudes of I.sup.2 t (amp.sup.2 sec) which in turn dictates the use of heavy mechanical switches with massive contact areas. Examples of such switches can be found in my Pat. Nos. 4,426,562 and 4,433,607.
The second switching system is a rotating pulse generator system in which an alternator of high short circuit current rating prestores kinetic energy and produces at its output terminals distinct and successive voltage pulses. In its simplest form, such a generator is connected to the breech ends of the projectile launching rails and if the breech electrical loop is shorted by the presence of a projectile package, that projectile will be fired because the voltage pulse in combination with the circuit and projectile rail parameters results in the desired and consistent accelerating current variation. In actuality, such a system will additionally require a series closing switch or switch array which closes the circuit to fire a projectile and which can or may open during the next current zero. This firing switch or switching system has to be extremely accurately timed so that accelerating current flow is started precisely at a desired point on the pulse voltage curve. As kinetic energy is depleted and the generator voltage drops, the timing of switch closure is repeatably adjusted so as to continuously result in a constant muzzle velocity for successive projectiles. A preferred switching system to accomplish this task would most likely utilize arrays of solid state devices. However, if solid state devices alone are used to form the switching system, an excessively large number of state-of-the-art devices would be required to handle the currents needed to produce acceptable acceleration of a practical projectile. Therefore, it is desirable to construct a switching system which takes advantage of the operating characteristics of solid state devices without requiring an excessive number of these devices.